Wide deployment of moving image communication, video distribution, and other new broadband services, using optical fiber communication that enables broadband and high-speed transmission, is anticipated. However, a functional (signal amplification) element, which, for example, corresponds to a triode transistor in electronics, that is, an optical functional element that performs signal amplification of optical signals by direct control by other optical signals has not been realized as of yet.
Thus presently, optical signals that have been transmitted at high speed are converted once into electrical signals, which are then subject to information processing in an electronic circuit, and the processed signals are converted back into and transmitted as optical signals. A limit is thus placed in the speed of signal processing due to the inability to directly control light by light. It is said that if signal processing can be performed on optical signals as they are, parallel processing will be enabled and further shortening of the processing time can be anticipated.
In this regard, the devices described in Document 1 or Document 2 are simply devices that switch light, in other words, gate switching devices that make use of wavelength conversion by Mach-Zehnder optical interferometry, and these devices had problems of being weak against temperature change and vibration and being strict in terms of setting conditions. Such conventional arts do not disclose anything in regard to arranging an optical signal amplifying triode, which, like a transistor in an electronic circuit, is equipped with a function of using input light as control light to obtain signal-amplified output light.
In the field of optical communication enabling broadband, high-speed, and high-capacity signal transmission, it is anticipated that communication, transfer, and distribution of optical signals be performed without degradation of the properties of high speed and high capacity. For an optical network based on wavelength division multiplexing (WDM), which is predicted to be constructed in the relatively near future, an optical signal transfer (optical signal relaying) art, of transferring wavelength division multiplexed optical signals, which are a plurality of types of laser light differing in wavelength and which have been transmitted from one optical transmission path, to desired optical transmission paths according to wavelength, will be important. In optical signal transfer for transferring an optical signal train (for example, a packet signal) that has been propagated via an optical fiber or other predetermined transmission path (for example, a wavelength bus) to other transmission paths indicated by labels, tags, or other routing information attached to the optical signal train, that is for example, in routing within an optical network or among optical networks, the high-capacity and high-speed characteristics of optical signal transmission must not be degraded and routers, that is, optical signal relay (transfer) devices are required to perform transfer processes at high-speed, be high in reliability, and be compact.
An optical path cross-connection device, such as that described in Document 3, has been proposed for this purpose. This device is equipped with a wavelength splitter, which splits a wavelength bus for wavelength multiplex transmission link into N wavelength group buses of G wavelengths each, and a routing processor, which executes a routing process on each of the wavelength groups split by the wavelength splitter, and is thus arranged to perform the routing process according to wavelength group. The routing processor of this optical path cross-connection device comprises a wavelength converter, which performs wavelength conversion according to each wavelength group, and an optical matrix switch, which distributes the wavelength-converted light and is controlled by a controller. This optical matrix switch is arranged with a mechanically-operated reflecting mirror switch that is positioned at the intersection of matrix-like optical paths and is alternatively operated by the controller to make one wavelength group, among the plurality of wavelength groups, be reflected by the reflecting mirror switch and thereby be output to a desired transmission path (paragraph 0042, FIG. 10(1)), or has an optical switch, which is alternatively operated by the controller, and mesh wiring and is arranged to make one wavelength group, among the plurality of wavelength groups, be transmitted by the optical switch and thereby be output to one transmission path inside the mesh wiring (paragraph 0043, FIG. 10(2)).
However, with the above-described conventional optical path cross-connection device, since the routing process is performed by the reflecting mirror switch or the optical switch, the operation of which is controlled by the controller, the switching operation of the reflecting mirror switch or the optical switch is performed in accordance with a command signal, which indicates the routing destination (destination) and is an output that is electronically processed at the controller. A portion of the optical signal thus had to be converted to an electrical signal, the destination information contained in the electrical signal, that is, a transfer-related signal included in a label or tag of a packet had to be extracted, and the optical signal had to transferred upon electrically controlling the operation of the reflecting mirror switch or the optical switch in accordance with the transfer-related signal. Thus, an adequate response speed could not be obtained. Also besides the above-described routing processor, since a wavelength converter, for performing wavelength conversion in accordance with the wavelength of the transmission path (wavelength bus) of the transfer destination, is equipped, and such a wavelength converter is disposed in addition to the routing processor, the device became large and in some cases, especially when a mechanically operated reflecting mirror switch is used, reliability could not be obtained.
Furthermore, in the field of optical communication enabling broadband, high-speed, and high-capacity signal transmission, it is anticipated that the identification, multiplexing and splitting, switching, and routing (transfer, distribution) of optical signals (optical data, such as packet signals) be performed without degrading the characteristics of broadband, high speed, and high capacity. In this field of optics, optical signal storage devices, which enable temporary storage and take-out at desired timings of optical signals, are generally demanded for optical signal processing systems that process optical signals and are represented, for example, by photonic router systems. This is because, just as memories are essential in signal processing in the field of electronics, optical signal storage devices, referred to as optical memories or optical buffers, are essential in the field of optical signal processing.
In this regard, optical memory devices, such as that described in Patent Document 1, have been proposed. With this device, a plurality of optical waveguide means 105 to 108, respectively arranged from optical fibers of different length in order to provide a plurality of types of delay times, are prepared, and arrangements to pass an optical signal through any of optical waveguide means 105 to 108 and enable storage of the optical signal by just the delay time corresponding to the propagation time in the corresponding optical waveguide means among optical waveguide means 105 to 108.
However, with this conventional optical memory device, the storage time of an optical signal is only determined in advance by the delay time corresponding to the propagation time in the optical waveguide means among optical waveguide means 105 to 108 through which the optical signal is made to propagate and the optical signal thus cannot be taken out at a desired timing. The degree of freedom of optical signal processing was thus limited and lowering of signal processing efficiency could not be avoided.
[Document 1] K. E. Stubkjaer, “Semiconductor optical amplifier-based all-optical gates for high-speed optical processing,” IEEE J. Quantum Electron., vol. 6, no. 6, pp. 1428-1435, November/December 2000.
[Document 2] T. Durhuus, C. Joergensen, B. Mikkelsen, R. J. S. Pedersen, and A. E. Stubkjaer, “All optical wavelength conversion by SOAs in a Mach-Zehnder configuration,” IEEE Photon. Technol. Lett., vol. 6, pp. 53-55, January 1994.
[Document 3] Japanese Published Unexamined Patent Application No. 2002-262319
[Document 4] Japanese Published Unexamined Patent Application No. Hei 8-204718
This invention has been made with the above circumstances as a background, and a first object thereof is to provide an optical signal amplifying triode that can perform an amplification process directly on optical signals by using control light. A second object is to provide an optical signal transfer method and an optical signal relay device, with which the routing of optical signals can be processed at high speed or by a compact device. A third object is to provide an optical signal storage device that enables storage of optical signals and taking out of the optical signals at an arbitrary time.
Upon carrying out various examinations with the above circumstances as the background, the present inventor found that in an optical amplifier, such as a semiconductor optical amplifier, a rare-earth-element-doped fiber amp, etc., spontaneously emitted light of peripheral wavelengths of an input light of a predetermined wavelength λ1 vary in intensity in response to intensity variations of the input light and this intensity variation varies inversely with respect to that of the signal intensity variation of the input light, and found a laser-induced signal enhancement effect, that is, a phenomenon wherein when laser light of another wavelength λ2 within the wavelength range of the spontaneously emitted light, that is, within the peripheral wavelength range of the input light is made incident upon being multiplexed with the input light, the overall intensity increases suddenly, with the signal (amplitude) variation of the spontaneously emitted light being maintained. The present inventor grasped this phenomenon as a wavelength conversion function from wavelength λ1 to λ2 and conceived an optical triode based on a tandem wavelength converter (All-Optical Triode Based on Tandem Wavelength Converter), with which this wavelength conversion is connected in two stages, and thus came to conceive an optical signal amplifying triode. A first aspect of this invention was made based on this knowledge.
The present inventor also noted that the optical amplifier of the above-mentioned optical signal amplifying triode not only has the function of wavelength conversion from wavelength λ1 to λ2 but is also a functional element equipped with the wavelength conversion function and a switching function and found that, by multiplexing optical signals with routing information by amplitude modulation, the functional element can be used favorably as a routing device, that is, a transfer device for wavelength multiplexed signals. A second and a third aspect of this invention was made based on this knowledge.
The present inventor also found that by making an optical amplifier of an optical signal amplifying triode, which exhibits the above-described phenomenon, perform the function of wavelength conversion from wavelength λ1 to λ2 and at the same time combining this optical amplifier with a wavelength splitter that performs distribution to different output transmission paths in accordance with the input wavelengths and interposing this combination in a ring transmission path in which optical signals circulate, the optical signals that are stored by being made to circulate can be taken out at an arbitrary timing. A fourth aspect of this invention was made based on this knowledge.